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Application Notes
High resolution images of patterned liquid surfaces have been acquired without inducing either capillary wetting (“bridging”) or strong perturbations of the liquid surface profile. Using a 5500 AFM system with Mac Mode® Controller from Keysight Technologies, Inc. in dynamic force mode an oscillating cantilever/tip ensemble is used to probe the liquid surface. In order to achieve high resolution stable images on these patterned liquid surfaces, it is crucial that the oscillating tip not contact the liquid surface. [1] The liquid topographies were imaged in non-contact, amplitude modulation AFM (NCAM-AFM), a dynamic imaging mode where the AFM cantilever is vibrated with relatively small amplitude Afr (1 to 10 nm) at a fixed frequency vo which is about 100 Hz higher than the fundamental cantilever resonance frequency. When a sharp tip is brought sufficiently close to the
surface, the attractive van der Waals force shifts the resonant frequency of the cantilever/tip ensemble to the left (decreasing). [2] Because the drive frequency of the cantilever is fixed, the amplitude of oscillation will decrease due to this shift. The software controls are set so that the approach terminates at a slightly reduced amplitude Asp compared to Afr. The AFM feedback mechanism is then activated in order to maintain this reduced amplitude Asp by adjusting the height of the tip relative to the sample surface. The tip is then raster scanned over the surface with the feedback loop enabled while monitoring the height of the tip. This data is then used to create a topographical image of the surface.
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